The present invention contemplates to improve the intake passage of an internal combustion engine and accordingly the characteristics of the internal combustion engine and further to enhance the charging efficiency of an air-fuel mixture during a slow running operation thereby to improve the output during the slow running operation.
In the internal combustion engine, generally speaking, the vacuum to be established in a combustion chamber or in a crank case during a suction stroke is applied to an intake passage thereby to suck an air-fuel mixture. When the intake stroke is ended, the intake passage is blocked by an intake valve from the combustion chamber or the crank case. As a result, there is established in the intake passage in the vicinity of the intake valve the so-called "intermittent flow" that the intake flow is interrupted while the intake valve is closed but is restored when the intake valve is opened. These phenomena raise no difficulty when the internal combustion engine is run with the intake flow regulating throttle valve having a larger opening than one half or run at a high speed even with the throttle valve having a smaller opening than one half. However, the phenomena remarkably deteriorate the performance of the internal combustion engine when this engine is run with the throttle valve having a lower opening than that, especially at a relatively slow speed.
More specifically, since the intake flow rate is generally so high in a large opening range of the throttle valve that the flow inertia is high, the intake air is at most compressed in the vicinity of the aforementioned intake valve even after this valve has been opened or closed but is steadily flowing upstream thereof. This means that any reduction in the charging efficiency is not invited by the closure of the intake valve so that no reduction takes place in the resultant output. In a low opening rate, however, since the intake air flow is restricted by the throttle valve, the flow inertia is not high. As a result, the intake air flow is stopped as the intake valve is closed to lower the charging efficiency so that the reduction in the output takes place.